My question is simple and probably very basic. I use dropbox and save most of my files on this cloud.
To find and access former files easily, I link them to a text in a sort of diary I write on Nisus Pro.
I realized recently that it is not possible anymore to access them directly. Nisus send a message saying that : "Nisus Writer Pro needs access to the "whatever" file. Would you like to grant permission by choosing the file?" Problem is that granting the permission means finding myself the file. And even though Nisus specify the " last known location", it is very inconvenient.
Here is my question : is there any possibility to grand Nisus and general access to folders in Dropbox in order to avoid having to deal with permissions for each file ?

I think I understand your question, but want to make sure everything is clear. If I understand correctly:

1. You have an older document that you created in Nisus Writer.
2. This document contains a lot of links to other files, links using file paths that were added before sandboxing.
3. Now you are using the current sandboxed version of Nisus Writer.
4. Every time you click on one of these older links, Nisus Writer asks you to select the target file so permission can be obtained.

This is annoying, so you wonder if there's any way to grant Nisus Writer access to all files in your Dropbox. The direct answer is that it is not possible to give Nisus Writer blanket access to a folder like your Dropbox. Under sandboxing each link to a file captures a security authorization just for that single file.

However, there is good news for you. It should be possible to write a macro that asks you for permission to the enclosing folder (eg: Dropbox) and then fixes all your links to use new sandboxing-aware bookmarks. I've written such a link conversion macro and posted it to our macro repository.